A Massachusetts-sized area in the northern Gulf of Mexico is no longer able to adequately support aquatic life due to low oxygen levels, and new research suggests this phenomenon might be linked to climate change.

The Gulf of Mexico’s Dead Zone is hypoxic, or characterized by very low oxygen, and some areas are anoxic, or without oxygen. These conditions are caused by excessive nutrient pollution, often from human activities such as agriculture. Nutrient pollution causes algae blooms to pollute the surface of the water and suck up oxygen. Weather, wind speed and direction, precipitation and temperature also influence the size of dead zones.

Geologists at Northwestern University's Department of Earth and Planetary Sciences are studying periods of low oxygen in the oceans that occurred as far back as 252 million years ago in the Mesozoic era.

Scientists study the Mesozoic (from 252 million to 66 million years ago), an era characterized by global temperature rise, to understand and make parallels to rising carbon dioxide levels today. Evidence of the effects of past warming can help scientists better predict future warming.

CO2 is a greenhouse gas that spreads through the atmosphere, holds in heat and is forcing global warming.

“There a real pressure to understand the relationship between CO2 levels in the atmosphere and climate,”said Brad Sageman, department chair of Northwestern’s Earth and Planetary Sciences department. “We need to be able to better predict what happens in the future as CO2 continues to rise and climate continues to warm.”

By analyzing data from rock cores extracted from thousands of feet below the ocean, scientists have been able to link these ocean anoxic events with a warming climate.

“Rock cores provide an excellent system to study in order to better understand the dynamics of how the earth system changes during perturbations in the earth’s climate,” Sageman said.

Seymour Schlanger, a former professor at Northwestern, first drilled deep sea cores and became fascinated by the intervals of rock highly enriched in organic carbon. The rock cores seemed to show a large flux of carbon to the sea floor, but scientists didn’t yet have enough data to explain the phenomenon, until they saw similarities in shallower rock near the surface.

The similarities found in younger rock led them to believe that these were periods of time when the whole ocean was anoxic, which would have been the result of a global event.

The cores they studied were rich in carbon and representative of a time when large amounts of organic matter, perhaps algae, near the surface of the ocean died and fell to the bottom of the ocean, where bacteria fed on them and depleted the water of oxygen.

More specifically, Sageman explained that warming drove higher rates of weathering on land, which released nutrients into the oceans that fed algae and allowed it to flourish. The resulting algae blooms had short lifespans and eventually fell to the bottom, through water deprived of oxygen. Bacteria in the ocean used sulfate in seawater to break down organic matter and the product of that is hydrogen sulfide, which reacts with any available iron and turn it into pyrite.

The rock cores that Sageman’s team studies are rich in pyrite and provide data on carbon, sulfur and iron levels during the Mesozoic.

Scientists have long theorized that the Mesozoic was characterized by the rather sudden onset of global warming from large volcanic eruptions, which emit a lot of CO2 into the atmosphere at once.

Today’s rising CO2 levels aren’t a result of volcanic activity, but from the burning of fossil fuels.

“Over the past few decades we’ve gotten increasingly good evidence that there’s a strong relationship between CO2 levels in the atmosphere and climate,” Sageman said, “and we believe this to be the consequence of [human-caused] activities.”